This invention relates to radio frequency transmitters, and in particular to facilitated tuning mechanisms for low power amplitude modulated radio frequency transmitters for use in transmitting information over a relatively short distance.
Low power radio transmitters have found wide application in the transmission of information over short distances, that is, a few hundred feet or less. One particularly advantageous application of such a transmitter is in the field of real estate sales. As indicated generally in Cooper, U.S. Pat. No. 4,541,119, and as shown in FIG. 1 hereof, it is known to provide a tape player 10 having pre-recorded information about a particular property 11, located within the house or other building 12 situated on that property. A radio transmitter 13, usually AM, is connected to the tape player 10 for transmitting the information on the tape via the airwaves 13a. A sign 14, usually on a signpost 14a, is generally placed on the property 11 indicating, generally among other things, the frequency at which the information is being transmitted. These transmissions may then be received on a car radio located in a passing car 16. Hence the driver 16a and any passengers 16b in the car 16 may receive and listen to the information being transmitted from the tape player 10 in the house 12, simply by tuning their car radio to the frequency indicated on the sign 14.
As this technique has come into wider use, however, certain problems have arisen. In particular, several of these tape players and transmitters, in combination, may be placed in homes in the same neighborhood, if they would happen to go up for sale at times coincident with each other. Alternatively, there may be other reasons for such transmitters to be located in proximity to each other. For instance, if the transmitters are rented for use in publicizing garage and rummage sales, a number of transmitters may be used in general proximity to each other. This proximity may cause interference between the various messages being transmitted, or simply between the transmissions themselves, if all the transmitters were to be set to transmit at the same single frequency. To avoid this interference on any particular frequency, the frequency at which the transmitters transmit must be adjustable or tunable to a large number of different frequencies.
However, up to the present time, tunable AM transmitters have either been simple to use, with a resultant extreme decrease in broadcast efficiency and range, or they have been extremely complex, and generally not suitable for tuning to different frequencies by the average layman not having specialized knowledge. The Cooper patent mentioned above indicates the problems that can be encountered in trying to get the signal from the transmitter to the receiver, and the solution there proposed is to move the transmitter closer toward the street to improve reception in the receivers in passing cars. What is needed in the marketplace, however, is an inexpensive AM transmitter that is easily tunable to broadcast over a large range of frequencies, and over a relatively large distance, such as over 100 feet using the unlicensed Federal Communications Commission (FCC) limitation of low power, in the range of 100 milliwatts, and short antennas, without the necessity of extensive technical knowledge on the part of the operator.
This invention relates to improvements to the apparatus described above and solutions to the problems raised thereby.